Originally Posted by elkston
If you are roleplaying a "good" Merrill who does not use Blood Magic, then ironically her rivalry bonus is the best. Being her friend only benefits her BM abilities which you do not utilize at all if you don't follow the "Blood of the Fist" tree. I used Merril as Entropy/Spirit/Nature person and she still kicked ass. This is due to her bonus damage resistance, the upgraded armor she gets if you romance her, and the true awesomeness of the "Walking Bomb" & Spirit Bolt spells.

Isabella is the least useful as a companion. She dies way to easily and there is also a very serious bug related to her Friendship bonus.

You can romance her and get the negative bonus? I noticed the armor change but not the stats change I forget compare. But also I had her at 100% friendship. Even if not roleplay and with the negative bonus, blood mage is certainly a good choice anyway. In fact blood mage for Hawke as first specialization to switch to cons upgrade instead of wiz is certainly a rather good build.

Dam I had forget the Isabella bug, crap seeing all those complain how weak she is have increased my wish to use her at next replay, something I had already in mind anyway. There's a manual fix for the bug alas I'm playing on Mac and use it will be a pain. the fix is quoted in the thread you link, it's the savegame editor of DAO working for DA2, be sure to get last version.